The present invention relates generally to antennas. More particularly, the present invention relates to a reverse-fed planar inverted F-type antenna (PIFA).
Each generation of communication devices is designed to be physically smaller than the previous generation. Small size is desirable to reduce physical size and weight and enhance user convenience. Many communication devices are designed and manufactured for consumer use. These include wireless devices such as radiotelephone handsets, handheld radios, personal digital assistants and lap top computers. Like all consumer products, these devices must be designed for low cost manufacturing and operation.
Manufacturers of wireless devices such as handsets, PDA's and laptops have very little room in their products given these extreme size and cost pressures. All of these devices require an antenna for wireless communication. These devices often need multiple antennas for operation at various frequency bands. It is desirable to incorporate the antenna within the package or case for reasons of esthetics, durability and size.
Such wireless devices typically pack a substantial amount of circuitry in a very small package. The circuitry may include a logic circuit board and an RF circuit board. The printed circuit board can be considered a radio frequency (RF) ground to the antenna, which is ideally contained in the case with the circuitry. Thus, the ideal antenna would be one that can be placed extremely close to such a ground plane and still operate efficiently without adverse effects such as frequency detuning, reduced bandwidth, or compromised efficiency. The antenna solution must also be cost effective for use in a consumer product.
A variety of other antennas having small profiles have been developed. These include Planar Inverted-F Antennas (PIFAs), types of shorted patches, and various derivatives which may contain meander lines. To date, however, none of these antennas satisfy the present design goals, which specify efficient, compact, low profile antennas whose height is at most λ/60 above a ground plane, where λ is the frequency of interest. There is a particular need for a 2.4 GHz antenna whose maximum height is at most 2 to 3 mm above a ground plane, and is thus well suited to devices requiring optimum performance in a compact volume, and operated according to the Bluetooth Standard, published by the Bluetooth Special Interest Group and IEEE Standard 802.11b, published by the Institute of Electrical and Electronic Engineers.
Devices for the Bluetooth Standard operate at 2.4 GHz (λ=125 mm). Existing shorted patch antennas are typically λ/8 to λ/4 in length. An antenna useful for such applications should have a length on the order of λ/10. One typical commercially available 2.4 GHz antenna is the SkyCross model 222-0463, available from SkyCross, Inc., Melbourne, Fla. This antenna has a volume of 3300 mm3. The antenna useful for these applications should have a volume under 300 mm3.
In addition to small size, portable devices typically are designed to be as lightweight as possible. Commercially available surface mountable 2.4 GHz antennas typically weigh 5 g or more. The SkyCross model 222-0463 has a mass of 8.9 g. The antenna useful for these applications has a mass under 1 g.
Cost must be reduced as well in these devices. Published embodiments of miniature patch antennas often use multiple layers of metal and multiple vias to create slow wave structures, such as meanderlines. One example is shown in U.S. Pat. No. 5,790,080. However, the antenna useful for these applications uses only one metal layer to construct the patch, and no vias, reducing fabrication costs.
Also, system designers want all components to be surface mountable to reduce assembly costs. But they also require low profile components to fit within available volumes. This problem is exacerbated when a ground plane is used under a surface mounted antenna, which is typically desired. Successful antennas need to be designed with the expectation of being surface mounted to a ground plane. A typical low profile 2.4 GHz antenna is the SkyCross model 222-0463, which is 3.56 mm in height. A lower height antenna is desired.